The trick is to have one side of the drive with the mounting brackets tightened and the other side basically floating, the screws would be in it just enough that they couldn't fall out. You may have to play with this a bit loosening or tightening the screws on the floating side until the vibrations have quieted to an acceptable noise level.

This is very effective with those monster 10K RPM SCSI drives. Also, the 3 1/2" to 5 1/4" conversion provides minor cooling benefits due to the increased airflow. Just cut a few holes in the bay cover, or even add a couple (quiet!) fans.

Edit: Zerotime has been kind enough to bring to my attention a comprehensive review at http://www.dansdata.com/novibes.htm, thanks!